The binding of the VP4 protein to MA 104 cells, a permissive host for rotavirus infection, was demonstrated using lysates of insect Spodoptera frugiperda (Sf9) cells that had been infected with a recombinant baculovirus expressing the VP4 gene of human rotavirus strain KU, DS-1, or 1076. Moreover, binding assays using the in vitro expressed VP5 or VP8 subunit of VP4, indicated that VP5 and not VP8 was responsible for the binding of the VP4 protein to a susceptible host cell.